Leanne Tonge

FZ3011 Assignment 1

FZ3011 Forensic Genetics

SECTION A

                                                                

Short Tandem Repeat Polymorphisms

Structure

STRs, also known as microsatellites or simple sequence repeats (SSR), are 2-6bp DNA sequences tandemly repeated units flanked by unique sequence. They are usually between 100-300 bp long. They are classified based on the structure of the number of core repeats (base pairs):

  • Dinucleotide - 2
  • Trinucleotide - 3
  • Tetranucleotide - 4
  • Pentanucleotide - 5 (Russell, 2006).

They are also divided into categories depending on the type of repeats:

  • Simple – contain the exact same sequence and length with non-consensus alleles.
  • Compound – contain repeats of two or more adjacent simple repeats (non-consensus).
  • Complex – can be of several repeat blocks, varying sequences or intervening sequences.
  • Hypervariable - complex repeats with numerous non-consensus alleles, e.g. STR loci SE33 or ACTBP2.
  • Microvariants – contain alleles with incomplete repeat units, e.g. allele 9.3 at TH01 locus (Butler, 2005).

Occurrence

They are scattered throughout the genome and occur approx. every 10,000 nucleotides (Butler, 2005).

Methods of Detection

  • Fluorescently labelled primers in PCR products are passed through Capillary Electrophoresis. As labelled PCR products migrate through the gel towards the anode on the laser they separate based on their size. Fluorescence is measured from exciting a dye molecule and detecting the light emitted from the excited dye. Automated and simple process, accurate but more expensive. Peaks seen in electropherograms allow for an easier interpretation of the data. E.g. ABI PRISM 3730xl has 96 capillaries and analyses 4000 a day.
  • Polyacrylamide Gel Electrophoresis (PAGE) – used to separate between 20-500 nucleotides long with a single base pair resolution (Butler, 2005).
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Uses in Forensic Science

They are highly polymorphic in a population so they are unique to an individual. As they are small and very useful in the forensic sense – many crime scene samples are trace evidence and sometimes very degraded. As the size of an STR is generally small they are analysed using PCR.  PCR can distinguish homozygotes and heterozygotes and define the copy number of each repeat (Russell, 2006), producing DNA profiles for comparison. The UK developed the National DNA Database in which STRs profile are stored for comparisons.

STR typing is less expensive than ...

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